This invention relates to a digital-to-analog converter (hereinafter referred to as a D-A converter) which provides an analog signal by adding together DC signals, each corresponding to the weight of one of digits of a digital input. More particularly, the invention pertains to a D-A converter which is capable of converting a digital input of many digits into analog form with high accuracy.
In a D-A converter widely employed in the past, use is made of various resistors for the purpose of voltage division, shunting, addition, amplification or like operations, but the accuracy of conversion is influenced by the accuracy of the resistance value of each resistor. To lessen such influence, a winding resistor which has a small temperature coefficient and which is pre-adjusted to have high accuracy is used; however, the winding resistor is bulky, hence is an obstacle to the reduction of the overall size of the converter and is not suitable for fabrication as an integrated circuit. From the viewpoint of integration, it is preferred to employ a thin film resistance element as the resistor, but such a resistance element has a large temperature coefficient and requires special equipment for adjustment of its resistance value.
Another conventional D-A converter is of the type that converts a digital signal to the duty ratio of a pulse signal and renders the converted output into a DC voltage to obtain an analog output. In this case, a clock signal is used for the conversion into the duty ratio and it is easy to obtain a clock signal which is highly accurate in period and highly stable with respect to temperature, etc. For rendering the converted signal into a DC signal, however, use is made of a smoothing circuit which causes a time lag in operation and hence makes high-speed operation possible.